In traditional wireless systems, including both second generation and third generation base stations, backhaul transport and baseband processing components are physically co-located with radio frequency processing components in a radio tower. This has not only driven up system maintenance costs, but also resulted in a rigid system that is difficult to adapt to quickly evolving radio standards. Recent innovations in wireless access networks have led to the introduction of distributed base stations and Remote Radio Heads (RRH). In this new architecture, centrally located base station “hotels” are used for backhaul transport and baseband processing, and they are remotely located from the radio towers. Very low loss optical fiber is then used to connect base station hotels with remote radio units. This distributed architecture promises to dramatically reduce costs associated with site acquisition, site leasing, and energy consumption for wireless systems.
Two competing open standards have been proposed by wireless industry to facilitate rapid adaptation of the remote radio head architecture: one is the Common Public Radio Initiative (CPRI) specification, the other is the Open Base Station Architecture Initiative (OBSAI) specification. Both standards allow flexible remote radio head topology, such as point-to-point, chain, ring, and tree. These different RRH topologies supported by CPRI are shown in FIG. 1. Such topologies have evolved to support multiple radio protocols, such as WCDMA, CDMA2000, and WiMax, etc. As new radio protocols emerge, these standards will be enhanced to embrace them to exploit the promised potential of the remote radio head architecture.
The RRH architecture also brings new design challenges for component providers. Chief among them is delay measurement and delay calibration from base station hotels to remote radio heads. The stringent requirements on delay measurement and delay calibration are driven from high-level system requirements. For example, third generation WCDMA systems require the air-frame timing among different antennas to be synchronized. When these antennas are arranged into RRHs through a chain or tree topology, fiber delay through CPRI/OBSAI links must be measured, and air-frames to each RRH must be aligned through delay calibration. Other system requirements such as location based service and transmission diversity also drive the required accuracy for delay measurement and calibration. For reduced system cost, it is highly desirable to have integrated solutions for the required delay measurement and calibration over CPRI/OBSAI links.
Both CPRI and OBSAI have defined delay measurement and calibration requirements over CPRI/OBSAI links for wireless systems employing RRH architecture. For clarity, the following discussion focuses on CPRI only, though the principles apply equally to OBSAI. In order to be compliant with high-level system requirements on user equipment positioning, CPRI requires that the full path round-trip delay measurement to have an accuracy of ±Tc/16, where Tc is the WCDMA chip period of 260.41 ns. Given that there are potentially multi hops in an end-to-end path in a chain or tree topology, and delay measurement errors accumulate with hop counts, this requires an integrated device to support delay measurement in the nanosecond range.
FIG. 2 shows the required delay measurement paths for a CPRI device in a chain topology. In addition, a CPRI device must also measure the external cable round trip delay from output port 5 to input port 4. The specific delay path measurements shown include: Δ1,2 and Δ4,5 which are the loop-back (digital) delay; Δ3,2 and Δ3,5 which are the add path (RF & digital) delay; Δ1,3 and Δ4,3 which are the drop path (RF & digital) delay; Δ1,5 which are the transmit signal (digital) through-path propagation delay; Δ4,2 which are the receive signal (digital) through-path propagation delay. Accordingly, there are a total of 9 delay paths to be measured for a CPRI device supporting chain topology. FIG. 3 shows a conceptual block diagram of a typical Nth CPRI device in such chain topology. For a CPRI device supporting tree topology with multiple CPRI links, the total number of delay paths can grow significantly. This large number of delay paths and the high measurement accuracy required present a challenging delay measurement design problem for creators of integrated CPRI devices.
In order to be compliant with high-level system requirements for transmit diversity and user equipment positioning, CPRI also requires that link delay excluding cable length to be accurate within ±Tc/32. In a typical implementation, the following factors contribute to device delay uncertainty: metastability effects when crossing asynchronous clock domains; phase uncertainty of recovered clocks from Serializer-Deserializer (SERDES) receivers; phase uncertainty of divided clocks; and delay variation over process, voltage, and temperature (PVT).
In general, delay uncertainties can be categorized into static uncertainty and dynamic uncertainty. Here, metastability effects and phase uncertainty of recovered clocks are un-predicable upon chip startup. However, with appropriate circuitry their contribution to device data-path delay can become fixed after system initialization and therefore will not change over time such that they can be considered to contribute to static uncertainty. On the other hand, delay variation over voltage and temperature changes over time contributes to dynamic uncertainty. Addressing both types of delays is therefore desirable.